European patent application No 0 249 230 relates to a single mode optical fibre having a zero dispersion wavelength in the wavelength range of 1500 nm-1600 nm, which optical fibre comprises a core consisting of an inner core of GeO2—SiO2 or GeO2—F—SiO2, and an outer core of F—SiO2, and a cladding of F—SiO2.
U.S. Pat. No. 5,848,215 relates to an optical fibre wherein the chromatic dispersion for the intended wavelength of 1550 nm of optical communications signals becomes zero over the entire fibre length as a result of the relative refractive index difference of the optical fibre and the core radius being increased or decreased in the same direction.
Such an optical fibre is furthermore known from European patent application No 0 785 448, which fibre must preferably meet a number of preconditions, in particular a/b<0.20 and b>15 μm. Such a fibre has the zero dispersion wavelength thereof within the range of at least 1560 nm, but not exceeding 1600 nm, which value is slightly shifted relative to the wavelength of the signal light (1.5 μm). The optical fibre that is known therefrom moreover has an MFD value not lower than 8.0 μm so as to decrease the optical power density without decreasing the signal intensity as a whole. In addition to that, the optical fibre that is known therefrom has a so-called cutoff wavelength of at least 1.0 μm but not more than 1.8 μm if the length thereof is 2 m. Moreover, all the examples described in European patent application No 0 785 448 exhibit a so-called zero dispersion higher than 1550 nm, which points to negative dispersion fibres. No mention is made of bending losses.
According to the usual method, an optical fibre having a predetermined external diameter is produced by heating one end of a bar-shaped preform and subsequently drawing the optical fibre from the plasticized end thereof. In an optical fibre that has been obtained in this manner, however, the cross-section of the core parts and the surrounding layers will exhibit a slightly ellipsoid or disturbed circular shape, which makes it difficult to obtain a perfectly circular concentric shape. Accordingly, the refractive index distribution in the direction of the diameter of the optical fibre thus obtained is not perfectly concentric, which leads to an increase of the so-called polarization mode dispersion (PMD). Thus, the “polarization mode dispersion” is a dispersion which occurs as a result of a difference in speed between two polarizations being orthogonal with respect to each other in a cross-sectional portion of an optical fibre. The influence of the aforesaid polarization mode dispersion is great if such optical fibres are used for long-distance transmission, which requires a large capacity over a long-distance. In addition, the influence of the polarization mode dispersion at high transmission rates per channel is considerable.
For some time optical fibres have been in use which are used in the transmission window at 1550 nm. These so-called dispersion shifted fibres (DSF) make use of the intrinsically low attenuation level of the fibre at wavelengths in the so-called C-band (1530-1565 nm), and in addition they have a shifted zero dispersion in the C-band so as to counteract widening of the transmission pulse due to chirping. Generally, Wavelength Division Multiplexing (WDM) is used for increasing the capacity of a glass fibre, in which several wavelengths in the same transmission window are processed for simultaneous transmission of data over a glass fibre. When WDM and high transmission rates are used, so-called non linear effects may have an adverse effect on the transmission capacity. These non-linear effects are Four Waving Mixing (FWM), Self-Phase Modulation (SPM), Cross Phase Modulation (XPM) and Parametric Gain (PG). Since FWM mainly occurs with zero or near zero dispersion, the known WDM fibres exhibit a low dispersion deviating from zero in the C-band. SPM is counteracted by increasing the effective fibre area. As a result, the intensity of the light to be transmitted in the fibre is reduced. One drawback of increasing the effective area is that the dispersion gradient of the fibre will increase. Consequently, the usability of the fibre at the edges of the transmission range is limited as a result of the dispersion being too high of too low. This leads to a serious limitation as regards the use of said fibres in transmission ranges outside the C-band, which ranges are being given a great deal of attention with a view to increasing the transmission capacity of a single fibre even further, such as the L-band (1565-1625 nm) or the S-band (1440-1530 nm). Solutions for the trade-off between dispersion gradient and effective area are thus sought in profiles in which the central portion of the light transmitting core has a lower refractive index, frequently in combination with an index ring in the cladding. Because of the large number of geometric properties that are to be controlled, such profiles are difficult to reproduce and to produce with a sufficient yield, however. Furthermore, the risk of deviations in the circular symmetry increases, which has an adverse effect on the Polarisation Mode Dispersion (PMD). In addition to that it is difficult to keep the bending losses, which play a role upon installation of a fibre, sufficiently low when using such profiles.